Clicking link in mail leads to messed-up Firefox

Don't know if this is a Firefox or Outlook problem. I have Office 07 and both Firefox 3.6.13 and 4.0b11 portable. I recently set Firefox 4 as my default browser. Now when I click a link in an email, a messed up version of Firefox 4 opens up - it appears to be trying to use the settings of the 3.6 version. Also, the link I clicked does not open up. If Firefox 4 is already open, the link opens normally in it. If I go to the Windows 7 default programs from the start button, it only lists one Firefox, but I don't know which version it is, but I am assuming it is 3.6 because 4 is installed in a nonstandard directory in the program directory (firefoxportabletest).
I am guessing that Outlook knows that Firefox 4 is the default browser, but is pulling the settings from the standard location for 3.6 for some reason. Any suggestions?

Outlook uses settings from the registry. While beta installations are supposed to leave the released version's setting undisturbed, it's possible they were disturbed. If you make IE your default browser temporarily, and then Fx3.6, perhaps that will fix it? Use the internal button in Firefox (under Tools > Options > Advanced > General > Check Now). Any luck?

I can do that and Firefox 3.6 displays and works correctly when I click a link in Outlook. But if I try to set IE then Firefox 4 as the default I still get the messed up UI. I don't know if I am ready to give up 3.6 yet, so I was going to try 4 as the default for a while, but can't seem to get it to work. I would rather not go to the extreme of uninstalling one or both versions and reinstalling 4, but that is beginning to look like that may be the case.

I can do that and Firefox 3.6 displays and works correctly when I click a link in Outlook. But if I try to set IE then Firefox 4 as the default I still get the messed up UI. I don't know if I am ready to give up 3.6 yet, so I was going to try 4 as the default for a while, but can't seem to get it to work. I would rather not go to the extreme of uninstalling one or both versions and reinstalling 4, but that is beginning to look like that may be the case.

I don't think it's a beta problem. I have FF 3.6.13 and 4.0b.11, using FF4 as the default browser, and I do not have this problem. When I click on a link in Outlook, FF 4 starts (if it is not running already) and displays the page normally, no problems. However, I am using Outlook 2003. Maybe there is an incompatibility with later versions of Outlook? Have you tried copying the link in Outlook, starting FF4 normally, and then pasting the link? Try that with Outlook still open, and with closing Outlook first. Could be a clue there?

As far as I know, FF3 and FF4 share the same profile, because in my case bookmarks, history, user changes in about:config etc. all show up the same in both versions. So probably not a configuration issue either.